'Cannot modify header information' Problem.

Try reviewing Solving “headers already sent” warnings

What exactly did you do?

Wow! Talk about obscure!

I managed to fix my problem by downloading (FTP) my twentyten/functions.php
I then edited it with a text editor (GEDIT) and saw nothing wrong – however I suspected bad chars at the beginning of the file and/or near the end of the first line where there was a ?> <?php

I carefully placed my cursor at the end of the first line of the file and moved it left until it was to the left of the <?php

I then backspaced up to and including the > of the ?> and then re-added the > and pressed the enter – all that to be sure no non-printing chars were between the ?> and the <?php

I then went to the first line of the file, placed the cursor well into the line and backed up to the < of the <?php – I pressed enter, then used ctl-shift-home to select everyting in the file from there to the top. (Should have been nothing, but apparently it wasn’t) – i deleted the selected “spaces”.

saving the file and uploading (FTP) back over the previous twentyten/functions.php then going to the login page on my browser.

All is well.

Much of this foolishness could be avoided by using a text editor which would show ALL characters, printing or otherwise.

tl;dr There were non-printing chars in the first line of my twentyten/functions.php

I did NOT put them there – never edited the file before – I have no idea how they got there. Not happy about that.

@mycupandchaucer – please start your own thread and post the actual error you are getting — it shows which file has the problem — so that makes it easier for someone to help you fix it.